The antenna structure of the present invention is intended to provide a means of accurately direction finding over broad, continuous, frequency spectrums. It is additionally intended that the antenna provide moderate gain, provide direction finding signals regardless of received signal polarization, be compatible with operation in side missile type radomes over broad frequency ranges, utilize a relatively simple phased array feed network and be inexpensive when manufactured in large quantities.
Although the antenna may be used for a wide variety of direction finding requirements, the antenna is especially applicable to the seeker requirements of an anti-radiation missile.
Reduced size log-permeable antennas in which selected elements are loaded to reduce the resonant frequency of the selected elements are not new, as exemplified by U.S. Pat. No. 3,543,277. However, the use of a uniformly tapered antenna structure wherein all of the elements are loaded in a constant manner, is new. As will be noted later, the purpose of the loading with this invention is to reduce the mutual coupling between the six log-periodic dipole antennas rather than to reduce the size.
Various configurations of cavity backed planar spirals, conical spirals and log-periodic dipole configurations have been utilized to date to achieve direction finding capability.
Multi-mode, cavity backed planar spirals require that absorbent filled cavities be included for milti-octave frequency coverage when using the antennas. The gain of the antenna is greatly reduced by the absorber. Also, since the lower frequency range to be received by the antenna determines the diameter of the antenna, it is required that the planar spiral be placed in a rearward position in the radome, making it particularly susceptible to radome internal reflections. For receiving signals of all polarizations, it is required that the spiral antenna be simultaneously excited from the center and the outer periphery. To do this over multi-octave bandwidths requires many spiral elements to eliminate undesired modes and therefore complex, expensive, feed networks. The fact that the relative phase of the sum and difference modes does not remain constant with changing frequency introduces additional feed network complexity, and loss of gain, for compensating the relative phase.
The conical spiral eliminates the backing cavity associated with the planar spiral and therefore has better gain. However, the compensation of the relative phase of the sum and difference modes is much more difficult than for the planar spirals. As with the planar spiral, reception of all polarizations requires simultaneous center and outer periphery feeding and the attendant feed network complexity. Additionally, the phase center of the sum is not coincidente with the phase center of the difference mode. This causes the relative phase of the sum and difference modes to vary as a function of the angle between the antenna boresight and the target. Complexity is introduced in compensating for this phenomena. The conical spiral functions fairly well within a radome since high frequencies radiate near the tip of the conical spiral and low frequencies toward the base of the cone. However, since the phase center of the difference mode is not as close to the tip of the cone as the sum mode, it is slightly more subject to radome internal reflections than the sum mode.
Interferometer arrays of planar spirals are not frequency independent and receive circular polarization of one sense only without complex feed network. Log-periodic dipole interferometers are linearly polarized. Conical spiral interferometers, when arrayed in a frequency independent manner, suffer aperture blockage and poor direction finding accuracy. Also, conical spiral interferometers receive circular polarization of one sense only without complexity. Phase sensing requires a minimum of three antenna elements for monopulse direction finding in two planes.
Amplitude sensing systems provide relatively poor direction finding accuracy. Measurement accuracy also requires component matching requirements.